Millions of gamers around the world pay real money for the credits in order to save hours of playing time.

Gold farming is rampant in China and other developing nations. Many Chinese gamers have full-time jobs as gold farmers but The Guardian story highlights the first time it has been practiced in labour camps.

Quoting figures from the China Internet Centre, the paper says almost £1.2 billion ($1.85 billion) worth of virtual currencies was traded in China in 2008.